This invention relates to a programmable frequency synthesizer and more particularly to such a programmable frequency synthesizer for use with such devices as radio receivers and transmitters.
Frequency synthesizers in the past have generally taken a form utilizing a plurality of groups of crystal controlled oscillators having preselected oscillators in each group providing output frequencies determined by the crystals together with mixers for summing the output frequencies in specified combinations to provide a desired synthesized output frequency. Sources of visible light have been used in the past together with a number of photo detectors and a card disposed therebetween having a predetermined pattern of punched holes therethrough for passing light to predetermined ones of the photo detectors and for blocking light from the remaining photo detectors. A digital output is thereby provided for selecting the appropriate crystal oscillators to provide the desired output frequency. Such a scheme is disclosed in U.S. Patent No. 3,531,724, issued to Fathauer.
Prior art devices have used mechanical means for moving the program card to provide different programs for disposition between a continuously energized light source and a single line of light transducers. The digital output obtained from the line of transducers has been utilized to select a predetermined combination of crystal oscillators having output frequencies which are mixed to obtain the desired output frequency. A matrix of diodes has also been used in printed circuit form for providing the frequency determinative output code. Devices are also known which continuously scan through a given band until a frequency is reached upon which a received signal is detected. Thereafter the scanning of the band is terminated and the device locks on the frequency of the received signal.
There is a need for a frequency synthesis apparatus which does not require crystal controlled oscillators and which may be programmed by merely forming a data mask having a predetermined series of areas for passing and blocking electromagnetic energy to provide a predetermined synthesized frequency.